1. Field of the Invention
The invention is directed to a method of fabricating a photonic crystal optical fiber or photonic band gap fiber, and in particular to a method providing independent control of pressures within a plurality of longitudinally-extending holes within a preform during the draw of the preform into the optical fiber.
2. Technical Background
Optic fibers are used in a wide variety of fields, including telecommunications, laser machining and welding, laser beam and power delivery, fiber lasers, etc. Typically, fibers are constructed from solid transparent materials such as glass and have a similar cross-sectional configuration along the length thereof. An alternative design to such fibers includes a microstructured optical fiber having holes or voids running longitudinally along the fiber axis. These holes generally contain air or an inert gas, but may also contain other materials.
Microstructured optical fibers may be designed to optimize a wide variety of properties, and are useful in numerous applications. For example, microstructured optical fibers may include solid glass core and a plurality of holes disposed in a cladding region around the core in a manner wherein the position and sizes of the holes are designed to yield dispersion values ranging between large negative values and large positive values. These particular fibers are useful in applications requiring dispersion compensation. Solid-core microstructured optical fibers may also be designed to provide a single mode wave guidance over a wide range of wavelengths. The majority of solid-core microstructured optical fibers guide light by a total internal reflection mechanism, wherein a low index of the associated holes act to lower the index of the cladding region in which they are disposed.
Another form of microstructured optical fibers includes photonic band gap fibers that guide light by a mechanism that is fundamentally different from the total internal reflection mechanism. Photonic band gap fibers have a photonic crystal structure formed in the cladding of the fiber, wherein the photonic crystal structure comprises a periodic array of holes. A core of the fiber is formed by a defect in the photonic crystal structure cladding. For example, the defect may include a hole of a substantially different size and/or shape than the holes of the photonic crystal structure. Typically, photonic band gap fibers are constructed with a hollow air core surrounded by a cladding structure that consists of a periodic array of air holes within the glass.
The photonic crystal structure of the microstructured optical fibers has a range of frequencies, known as the band gap, within which light cannot propagate within the photonic crystal structure. In application, light introduced into the core of the fiber having a frequency within the band gap will not propagate in the photonic crystal cladding, and will therefore be confined within the core. A photonic band gap fiber may have a core that is formed from a hole larger than those of the photonic crystal structure. The key aspect of the hollow core photonic band gap technology is the production of a fiber with an air core having a low non-linearity and low attenuation. Specifically, the light is guided within a gaseous medium, lowering the losses due to absorption and rayleigh scattering associated with the glass materials. As light is guided in a gaseous medium, the hollow-core fiber may be constructed to provide extremely low non-linearity. Moreover, hollow-core microstructured optical fibers are well-suited for guiding light over a very broad range of wavelengths. Advantages of such a fiber include the application within high power transmission at wavelengths of from UV to IR range, such as within welding, lithography, cutting industries, and the like, and also for applications requiring ultra-low loss transmission of telecommunication signals.
Microstructured optical fibers are fabricated using methods roughly analogous to the manufacture of all-glass optical fibers. A preform having the desired arrangement of holes is formed, then drawn into fiber using heat and tension. During the drawing process, the size, shape and arrangement of the holes may be significantly distorted depending on the viscosity of the material and surface tension within the holes. Such distortions are especially damaging in hollow-core photonic band gap fibers, as the band gap may be quite sensitive to variations in characteristic dimensions of the photonic crystal structure such as hole size, pitch and symmetry. Such distortions can also affect the geometry of the core/cladding boundary which can in turn have a significant effect on the attenuation behavior of the guided mode.
Heretofore, the manufacturing process utilized to manufacture hollow-core microstructured optical fibers have been difficult to reproduce, relatively expensive, and time consuming. As the skilled artisan will appreciate, the holes of the preform used to make microstructured optical fibers can be quite small (e.g. less than a few hundred microns in diameter), and coupling the numerous holes of the microstructured optical fiber preform to different pressure systems is not a trivial task. A method is desired that enables improved control of the fiber geometry and yet is still practical, robust and repeatable, thereby reducing the overall time, cost and complexity associated with the manufacture of hollow-core microstructured optical fibers.